The invention relates to a process for the manufacture of alkali metal hydroxide.
It relates more particularly to a process for the manufacture of alkali metal hydroxide, especially of sodium hydroxide, by electrodialysis of an aqueous solution of alkali metal halide in an electrodialysis cell.
U.S. Pat. No. 4,592,817 describes a process for the manufacture of aqueous alkali metal hydroxide solutions by electrodialysis of aqueous alkali metal chloride solutions in an electrodialysis cell of the type containing three compartments which are defined respectively between an anionic membrane, a cationic membrane and a bipolar membrane which is inserted between the anionic membrane and the cationic membrane. To this end, an aqueous alkali metal chloride solution is introduced into the saline compartment of the cell, delimited between the anionic membrane and the cationic membrane, and, on the one hand, an aqueous alkali metal hydroxide solution is collected from the alkaline compartment, delimited between the cationic membrane and an anionic face of the bipolar membrane, and, on the other hand, an aqueous hydrochloric acid solution is collected from the acidic compartment, delimited between the anionic membrane and a cationic face of the bipolar membrane.
In this known process, the aqueous alkali metal hydroxide solution which is collected from the electrodialysis cell is diluted by the water of hydration of the alkali metal cations which cross the cationic membrane. The concentration of the aqueous alkali metal hydroxide solution will consequently depend on the concentration of the alkali metal chloride solution used. Substantially saturated alkali metal chloride solutions are usually used. However, it is observed in practice that the aqueous alkali metal hydroxide solutions obtained in this known process have a water content which is substantially higher than that predicted on the basis of the concentration of the alkali metal chloride solution and on the basis of the operating conditions of the electrodialysis cell, which may constitute a drawback.
The invention tends to overcome this drawback of the known process described above, by providing an improved process which enables more concentrated aqueous alkali metal hydroxide solutions to be obtained.
Consequently, the invention relates to a process for the manufacture of alkali metal hydroxide, according to which an electrodialysis cell containing three compartments is used, an aqueous alkali metal halide solution is circulated in a saline compartment of the cell, delimited between an anionic membrane and a cationic membrane, and an aqueous alkali metal hydroxide solution is extracted from an alkaline compartment of the cell, delimited between the cationic membrane and an anionic face of a bipolar membrane; according to the invention, an alkali metal halide is introduced into an acidic compartment of the cell, which is delimited between the anionic membrane and a cationic face of the bipolar membrane.